1. Field of the Invention
This invention relates to machines for machining spherical objects, e.g. metal or plastics material balls for anti-friction bearings or other uses. Particularly, the invention is concerned with vertical plate machines, in which the objects are machined between opposed generally vertical faces of opposed relatively rotatable working plates.
2. Description of the Prior Art
Bearing balls of highly exact sphericity are made in a number of stages. Firstly the blanks are headed or rolled to a rough spherical shape which leaves certain portions projecting from the material. The balls are then machined to the desired spherical shape in several stages on special machines, usually with heat treatment in a furnace after the first or second stage as far as metal balls are concerned. The first stage may be "flashing" in which metal balls are ground between two very hard "Meehanite" plates, the second may be grinding in which one grinding plate and one "Meehanite" plate are used and the third stage may be progressive lapping operations in which the balls are polished between two special metal plates and in which grit and coolant are used in most cases.
With the progress made during the last ten years in the techniques of rolling the roughly shaped steel balls at very high production rates compared to the conventional heading techniques, it has become very important also to improve the production rates of those machines engaged on processing the steel balls subsequent to rolling.
Previously conventional vertical plate machines used for these machining operations have had two members (in the form of castings) carrying opposed machining plates one or both of which have a number of grooves in them and one of which is rotated while the other is stationary, pressure being exerted on the stationary member to urge it towards the rotating one. The stationary member and its plate have a gap through which the balls are fed to and from the plate grooves. The balls are ground as they are driven around between the plates, which they hold apart. The very high pressure used, e.g. up to about 80,000 lbs. (36,000 Kg) for the flashing operation, necessitates very high rigidity in the frame which supports both members and receives the reaction forces from the stationary member and from the thrust bearing for the rotating member. Large and expensive cast iron frames are used, and these frames leave little room for the apparatus for delivering the balls to and receiving them from the grooves in the plates. In spite of the size of frame employed, there is a strong tendency for the frame to distort which leads to uneven wear of the grooves of the stationary plate. FIG. 1 of the drawings is a simplified view of a conventional machine of this kind and will be more fully described below.
A known development of the machine illustrated in FIG. 1 which greatly increases its output but with a similar increase in size and cost, is to provide a second pair of plate-carrying members axially spaced from the first pair, the central two members of the four plate-carrying members thus provided being supported in common by a single central pillar of the frame of the machine and the outer two members each being supported by a further pillar. The forces applied to the central pillar from the central two members at least to some extent counter-balance each other. The machine is very large and cumbersome.
An especial disadvantage of these known vertical plate machines is the difficulty of access to the plates for inspection for wear and for replacement. The presence of the large pillars close behind the plate-carrying members prevents movement of the members a sufficient distance apart for easy access to the plates. A periscope arrangement has been needed to inspect the faces of the plates, and changing plates is a troublesome operation.
Apart from vertical plate machines, there are also machines of a different type in which the working faces of the working plates are horizontal. These machines are generally smaller than vertical plate machines. They are not capable of producing steel balls of highly accurate sphericity at high outputs particularly because of the problems encountered in feeding the balls to and from the grooves in the plates. Because the plates are horizontal, the balls collect in large numbers in the feeding gap in the stationary plate, so that collisions of considerable force occur between balls when a ball is ejected at speed from between the plates. Collisions between balls reduce the accuracy of their macrofinish.
Another disadvantage of horizontal plate machines is also poor access to the plate surfaces, since the plates and their associated carriers, which are heavy, must be lifted in order to gain such access.
In one known horizontal plate machine, made by Sebastian Messerschmidt Spezialmaschinenfabrik, the stationary plate carrier is mounted above the rotary plate carrier and the reaction load from the plates is transmitted by a tensioned rod passing centrally through the plates. The load is applied by the rod to the frame of the machine, in which the rotary plate carrier is mounted in ball bearings the races of which are coaxial with the axis of rotation. The rotary plate carrier is driven through a gear engaging toothing on an axial end face of an annular sleeve secured to the back of the rotary plate carrier. This machine suffers from the disadvantages of its type described above.